The fuel flow of utility engine carburetors is a very small volume as compared to the fuel flow volume of a four-cycle automobile carburetor. Relatively big variations in fuel flow occur due to differences in dimensional accuracy and part quality within the carburetors. Also, engines that receive fuel via a carburetor have different individual performance characteristics. Therefore, adjustments are needed to correct fuel flow for each engine and each carburetor.
For the above reasons, a diaphragm or float type carburetor can adjust the fuel flow with a manual adjust valve by changing the effective area of passage to the main nozzle, the slow port, or the idle port, from the metering chamber. (U.S. Pat. No. 3,404,872, Japan 47-21848).
The manual adjust valves comprise a needle shaped design that changes the effective area of fuel passage proportionally to the rotation of a male screw that is installed within a female receptacle of the carburetor body. A valve head, that projects outside of the carburetor, is used to rotate the screw, and thus adjust the tip of the valve needle inside the carburetor from the outside of the carburetor. Basically, the engine and carburetor manufacturer, along with the consumer operator, can adjust the fuel flow by adjusting the manual adjust valve.
However, most consumer operators are not familiar with the delicate operation of a manual adjust valve. The instructions say "don't adjust slow speed adjust valve for idle and starting." Yet, in many cases the main adjust screw is reset by a consumer usually trying to improve the temporary poor performance of the engine due to the effects of temperature and altitude. As a result, difficulties arise such as the engine looses power, the engine will not start, or the engine exhaust composition worsens.
Moreover, emission regulations have recently been enacted for utility engines that require that the consumer not be able to operate the adjust valve beyond the limited range set by the manufacturer. Therefore, the regulations require a limit device that allows the consumer to operate the adjust valve within the emission regulation limits. In addition, the regulations require a design for a limit device that is very difficult to remove.
Currently, the limit device for the adjust valves includes a cap with a flange. The cap is installed on the head part of the needle by a tight fit. The slow adjust valve acts as a stopper for the flange of the cap. However, the current cap design can be removed from the head part by pulling in the axial direction exposing the end of the head part, thereby making it possible to rotate the adjust valve beyond the required limits. Even the flange contact stopper is easy to remove. Therefore, it would be desirable to have a reliable limit device that does not allow the consumer to operate the adjust needle beyond the emission regulations.